You have pressure balanced shower valves. If one side has no pressure, then the opposing side matches that.
So...........when you turned off the hot, it matched that and turned down the cold too.
When the water heater is turned back on, the pressure will come back.

In the meantime, if you want cold showers at least, you can loop a flex between the shutoff and the outgoing pipe and take the water heater out of the loop.

You turned off water to the water heater right? Right now it's connected by two pipes, maybe flexes. If you want water to the hot side of your faucets, you will need to continue feeding the hot side from the existing supply that was used for the water heater.

You turned off water to the water heater right? Right now it's connected by two pipes, maybe flexes. If you want water to the hot side of your faucets, you will need to continue feeding the hot side from the existing supply that was used for the water heater.

Alternatively you cold buy a 3/4 inch NPT nipple. It can be brass, galvanized, or schedule 80 CPVC. Normally you would not want to use galvanized, but you will discard this soon anyway. The length is not important, as long as you can unscrew both lines from the WH, and screw them to the nipple.

Either way works, so do what seems easier to you.

Keep that valve closed until you get the nipple in place. Use Teflon tape on the threads. You would want to turn off the gas to the WH.